The treatment of metal surfaces with solutions to provide corrosion resistance and to enhance the adhesion properties of the surface are well known in the art. As used herein metal surfaces refers to metal in general such as steel, galvanized steel, titanium, copper, zinc and in particular aluminum. The term aluminum as used herein refers to the pure metal as well as alloys thereof. Compositions are applied to metal surfaces to, in part, improve the adhesive properties of a subsequently applied coating such as paint, varnish, lacquer or adhesives. Such coating may improve either the initial adhesion of the subsequently applied coating to the surface or the maintenance of such adhesive properties over time, or both the initial adhesion and maintenance of adhesion properties.
U.S. Pat. No. 5,514,211 discloses an aqueous composition for the treatment of metal surfaces which comprises a tetraalkyl silicate or a monomeric or oligomeric hydrolysis product thereof, and an inorganic passenger powder below 100 nanometers in diameter, such as fumed silica, both dispersed in a fluid aqueous medium. The coating composition disclosed in U.S. Pat. No. 5,514,211 provides enhanced adhesion properties when applied to a metal surface. However, the hydrolyzed tetraalkyl silicate/colloidal silica aqueous dispersions disclosed therein exhibit a lack of stability. That is, such aqueous dispersions are subject to gelling and sludging shortly after preparation. The formation of a hard packed sludge, which is difficult to redisperse, is not uncommon. Such gelling and sludge formation and the resulting required remixing prior to application adversely affected the marketability of the surface treatment composition disclosed therein.
The present inventors have discovered that the addition of a stabilizer to the aqueous dispersion metal treatments disclosed in U.S. Pat. No. 5,514,211 significantly enhances the stability of the dispersion without adversely impacting the metal surface treatment properties of the dispersion. The dispersion stabilizer of the present invention is preferably a high molecular weight carboxylic acid polymer, copolymer or salt thereof, more preferably the dispersion stabilizer of the present invention is a high molecular weight acrylic acid polymer, copolymer or salt thereof, and most preferably a high molecular weight polyacrylic acid.